First things first:
where does that incorrection come from
When I was at school we were taught never to end a sentence with a preposition. The teacher demonstrated the error thus: "Never use a preposition to end a sentence with!"
Next, we Geordies would say "Agorrit off (or on) the internet." The word "agorrit" is Geordie and not, as some would claim, the third person singular of the latin verb "agorro" which doesn't exist. Furthermore, the word "agorrabit" is not the future imperfect tense of the same verb!
To return to the original question, what is the correct preposition to use in the sentence "I bought it --- the internet"? We buy things OFF shelves or the backs of lorries, FROM shops or salesmen, and, curiously, OVER the phone. I suppose that if the phone happens to be sitting in front of you on a shop counter then that last one would be correct! Some people do buy stuff ON, OFF, OVER or THROUGH the internet. If you want to be precise about what actually happens, you buy stuff BY WAY OF or USING the internet - and the same goes for the phone, by which we mean TELEphone.
Language evolves and so words like phone, vet, zoo and TV become standard English. (Try finding "sexy" in an old dictionary!) On the other hand, the phase OFF OF the internet is just plain MINGING.
PS: Is "ming" a verb as in "You ming." are can it only be used in the adjectival form "You're minging."?